As is known in the art, peroxide curable rubber compounds offer several advantages over conventional, sulfur-curing, systems. Typically, these compounds display very fast cure rates and the resulting cured articles tend to possess excellent heat resistance and low compression set. In addition, peroxide-curable formulations are much “cleaner” in that they do not contain any extractable inorganic impurities (e.g. sulfur). Such rubber articles can therefore be used, for example, in condenser caps, biomedical devices, pharmaceutical devices (stoppers in medicine-containing vials, plungers in syringes) and possibly in seals for fuel cells.
The use of butyl-type rubber for sealing applications over other synthetic rubbers is preferred because of butyl rubbers non-permeability of gases such as oxygen, nitrogen, etc., and moisture and its stability to acids, alkalis and chemicals.
A commercially available butyl terpolymer based on isobutylene, isoprene and divinylbenzene (DVB), sold under the tradename Bayer XL-10000, is curable with peroxides alone. However, this material possesses some disadvantages. Since the DVB is incorporated during the polymerization process a significant amount of crosslinking occurs during manufacturing. The resulting high Mooney viscosity (ca. 60-75 MU, ML1+8@125° C.) and presence of gel particles make this material difficult to process. Also, the presence of significant amounts of free DVB can present safety concerns. It would be desirable to have an isobutylene based polymer which is peroxide curable, completely soluble (i.e. gel free) and devoid of harmful or malodorous chemicals in its composition.
It is well known that compounds containing butyl rubber and polyisobutylene decompose under the action of organic peroxides. Therefore, in these compounds the presence of cure promoters (co-agents) is needed.
One approach to obtaining a peroxide-curable butyl-based formulation lies in the use of conventional butyl rubber in conjunction with a vinyl aromatic compound like DVB and an organic peroxide (see JP-A-107738/1994). In place of DVB, an electron-withdrawing group-containing polyfunctional monomer (ethylene dimethacrylate, trimethylolpropane triacrylate, N,N′-m-phenylene dimaleimide) can also be used (see JP-A-172547/1994).
White et al. (U.S. Pat. No. 5,578,682) discloses a process for obtaining an uncured polymer with a bimodal molecular weight distribution derived from a polymer that originally possessed a monomodal molecular weight distribution. The polymer, e.g., polyisobutylene, a butyl rubber or a copolymer of isobutylene and para-methylstyrene, was mixed with a polyunsaturated crosslinking agent (and, optionally, a free radical initiator) and subjected to high shearing mixing conditions in the presence of organic peroxide. This bimodalization was a consequence of the coupling of some of the free-radical degraded polymer chains at the unsaturation present in the crosslinking co-agent. White, et al. is silent about the filled compounds of such modified polymers or the cure state of such compounds.
Sudo et. al. (U.S. Pat. No. 54,465) discloses a method for curing regular butyl, with isoprene contents ranging from 0.5 to 2.5 mol %, by treatment with a peroxide and a bismaleimide species. The rubber composition contains optionally an organosilicone compound and the articles thereof are useful for pharmaceutical chemicals or medical treatments. The compositions of Sudo, et al. have excellent molten fluidity after cure.
Co-Pending CA Patent Application 2,458,741 describes the preparation of butyl-based, peroxide curable compounds which employed the use of novel grades of high isoprene butyl rubber. According to this application, N,N′-m-phenylenedimaleimide is useful as a cure promoter (co-agent).
Cotsakis et al. (U.S. Pat. No. 6,120,869) discloses a pressure sensitive tape for forming water-tight field joints in rubber membranes. This adhesive roofing tape was based on a combination of brominated butyl rubber and EPDM rubber utilizing a peroxide cure system. Both these rubbers can be cured separately with peroxides alone. An important aspect of Costsakis, et al. is to have a high molecular weight polyisobutylene as a plasticizer. The degradation products from the action of peroxide on PIB contributed to surface tack.
For some specific applications, like elastic closures for electrolytic condensers (capacitors) the presence of halogens in the compound is not desirable. This is because the halogens present in the elastic rubber cap (in contact with an electrolyte) can interact with a copper wire of the condenser causing corrosion and subsequently electrolyte leakage. Therefore the above-mentioned applications would not be suitable for condenser caps.
Walker et al. (U.S. Pat. No. 3,584,080) claimed peroxide-vulcanizable compositions comprising copolymers of an isoolefin (e.g., isobutylene) and an aromatic divinyl compound (like divinylbenzene) together with a minor amount of a rubbery or resinous polymer (such as PE, NR or EP(D)M rubber) present in a mixed compound. The central aspect of this invention was that each kind of rubber introduced into a compound was peroxide-curable on its own. Both isobutylene-divinylbenzene copolymers and isobutylene-isoprene-divinylbenzene terpolymers are peroxide curable. The present invention differs from that of Walker et al. in that no divinylbenzene is present in the isobutylene-containing polymer (i.e., butyl rubber). Surprisingly, the properties of the inventive compounds of the present disclosure gave good final properties after curing, despite the fact that butyl rubber on its own degrades under the action of free radicals. It is much easier to manufacture a regular butyl rubber than butyl rubber crosslinked with divinylbenzene.
Saotome (JP 55-62943 A1) discloses a thermoplastic elastic polymer composition produced by heating and mixing a mixture of butyl-based rubber (IIR or PIB) and an EP(D)M rubber in the presence of an organic peroxide, and partially curing the mixture. The resulting polymer composition has excellent molten fluidity and is intended for hot-melt adhesives and sealants when a tackifier is added to it. The amount of peroxide present in the compound is typically in a range of 0.1 to 1.5 parts per 100 parts of polymer. The examples are based on blends composed of 70 parts of EP(D)M rubber and 30 parts of butyl rubber or PIB. Saotome is specific for compositions having excellent molten processability (and hence the degree of crosslinking has to be limited) and it is silent of the cure state characteristics (e.g., from the MDR test) of the compounds. In fact, the central aspect of Saotome is to suppress the generation of gel which hinders the processability in melt.
EPDM rubber is known to be used in several applications where butyl rubber is utilized, e.g., cable insulation, shock absorber parts, window seals, roofing membranes and condenser caps. However, EPDM cannot match butyl rubber in impermeability for gases and moisture. For a specific application like a condenser cap, a peroxide cured compound based on butyl and EPDM should be superior to that containing EPDM alone.
The present invention describes the preparation of butyl-containing, peroxide-curable compounds which employ the use of regular butyl rubber and EP(D)M rubber. The butyl rubber gives predominantly a sealing property and surprisingly the EP(D)M rubber acts as a cure promoter (co-agent) for IIR. Since the co-agent is polymeric in nature it minimizes the co-agent leaching, a common problem when utilizing low molecular weight additives.
The MDR and stress-strain characteristics of the vulcanized compounds prepared according to the present invention are comparable or better than those of a comparative compound based on a peroxide-curable butyl rubber Bayer XL-10000. This demonstrates a significant degree of crosslinking in the final products which are not intended for good processability in the molten state, as in JP 55-62943 A1. The rubber articles based on compounds according to the present invention are useful for sealing applications where a high Shore A hardness, good elongation and low permeability to gases or moisture is important such as for electrolytic condenser caps.